When you activate a new phone (or slip your SIM card into a new phone), your old phone ceases to have cell and data access—though it can still make emergency calls. So, if you have an old phone that's sitting around doing nothing, you might as well stash it in your glove box, the pocket of your big winter jacket, or wherever else you can think of. That way, if you get into a situation and you've lost your phone, left it somewhere else, or it's just plain run out of battery, you can still make a 911 call if you need to. Heck, you can even use this if you've repurposed your phone as, say, a music player for your car—just remember that no matter what you've repurposed it to do, it can always be an emergency phone as well.